let fs = require('fs');
let buf = '';

process.stdin.on('readable', function() {
    let chunk = process.stdin.read();
    if (chunk) buf += chunk.toString();
});

process.stdin.on('end', function() {
    let hash = {};
    let s = buf.split("\n")[0];
    for (let c of s) {
        if (c in hash) hash[c]++;
        else hash[c] = 1;
    }
    
    for (let c in hash) {
        if (hash[c] === 1) {
            console.log(c);
            return;
        }
    }
    console.log("no");
});